1. Field of the Invention
This invention relates to the field of subsurface reservoir development, to a method of geosteering and for a system predicting the subsurface environment ahead of a drill bit. The methods and systems are related generally to drilling and measurement systems and more particularly to a system of drilling boreholes having a measurement-while-drilling or logging-while-drilling (MWD or LWD) tool wherein the tool measurements are used to predict and guide the drilling operation.
2. Description of the Related Art
In the oil and gas industry, those skilled in the art have long recognized the importance of obtaining various borehole measurements during the course of a drilling operation. Typically, these measurements include such data as the weight imposed on the drill bit, the torque applied to the drill string, the inclination and azimuthal direction of the borehole interval that is then being drilled, borehole pressures and temperatures, drilling mud conditions as well as formation parameters including, but not limited to, resistivity and natural gamma emission of the earth formations being penetrated. Heretofore most of these measurements were obtained either by temporarily positioning special measuring devices in the drill string or by periodically removing the drill string and employing suitable wireline logging tools.
In recent years, however, the drilling technology has advanced sufficiently that these measurements can now be readily obtained by so-called measurement-while-drilling or “MWD” tools that are tandemly coupled in the drill string and operated during the drilling operation. Several MWD tools presently in commercial operation typically include a thick-walled tubular body carrying various sensors and their associated measurement-encoding circuitry often positioned in the drill string just above the drill bit for measuring the conditions near the bottom of the borehole. These commercial tools generally employ a selectively-operable acoustic signaler which is cooperatively arranged in the tool body for successively transmitting encoded measurement signals through the drilling mud and/or within the drill string to the surface where the signals are detected and recorded by suitable surface instrumentation.
The typical commercial MWD tool is arranged as a multi-sectional tool having various special-purpose modules that are respectively housed in separable thick-walled bodies and suitably arranged to be coupled together in various combinations for assembling an MWD tool capable of obtaining one or more selected measurements. The multiple sections require both mechanical and electrical connections, such as the prior art arrangement shown in FIG. 1. The illustrated components, known in the prior art, include transmitters and sensors for determining downhole formation characteristics. The prior art methods and apparatus include downhole tools comprising acoustic signal transmitters and sensors to determine, for example, subsurface formation velocity as the tool traverses the formation. This type of measurement does not provide for determining an image of subsurface formation reflectors before the drill bit has reached the reflectors. Downhole tools comprise one more sensor subs for data acquisition and are not limited to gamma ray and resistivity methods.
U.S. Pat. No. 6,088,294 to Legget et al, discloses an invention that provides a closed-loop system for drilling boreholes. The system includes a drill string having a drill bit and a downhole subassembly having a plurality of sensors and MWD devices, a downhole computing system and a two-way telemetry system for computing downhole bed boundary information relative to the downhole subassembly. The downhole subassembly includes an acoustic MWD system which contains a first set of acoustic sensors for determining the formation acoustic velocities during drilling of the wellbore and a second set of acoustic sensors that utilizes the acoustic velocities measured by the system for determining bed boundaries around the downhole subassembly. A computing system is provided within the downhole subassembly which processes downhole sensor information and computes the various parameters of interest including the bed boundaries, during drilling of the wellbore. In one embodiment, the first and second sets (arrangements) of acoustic sensors contain a transmitter and a receiver array, wherein the transmitter and some of the receivers in the receiver array are common to both sets of acoustic sensors. Each receiver in the receiver array further may contain one or more individual acoustic sensors. In one configuration, the distance between the transmitter and the farthest receiver in one of the acoustic sensor sets is substantially greater than the distance between the transmitter and center of the receivers in the second set. The downhole computing system contains programmed instructions, models, algorithms and other information, including information from prior drilled boreholes, geological information about the subsurface formations and the borehole drill path. This invention is directed to determining formation boundaries adjacent (normal) to the logging tool.
In drilling a borehole to recover oil from the earth, it is often helpful to turn or steer the downhole drill bit toward, away, from or through subterranean targets. To facilitate steering drillers need to know the subsurface drill bit location. The location of targets ahead of the bit may also be required, as well as some warning or indication of drilling hazards such as over-pressured formations or thin, shallow gas intervals. Surface seismic surveys generally include this type of information, but resolution and depth location of these data may be poor because surface seismic surveys are time based (rather than depth based) and the low frequencies generally available are not conducive for high resolution. For example, to determine the depth of a reflection, a speed of sound for the earth formation must be known. Consequently, these systems require depth calibration to accurately determine locations of target horizons or drilling hazards. Traditionally, this calibration has been provided by either offset well sonic data or wireline checkshot data in the current well. Offset data is often inadequate however due to horizontal variations in stratigraphy between wells.
There is a need to be able to plan complex well paths through subsurface reservoirs for optimal placement to the most productive locations. There is a need for a method and system of receiving data from the vicinity of the drilling string generally and the drill bit particularly without the need to halt drilling operations, and to model and predict accurately the subsurface reservoir conditions. Additionally, there is a need to vary such models and predictions as actual data from the subsurface drill path becomes available.
The methods of the present invention overcome the foregoing disadvantages of the prior art by providing systems and methods for modeling and predicting subsurface conditions and adapting the model with subsurface data as the data are acquired as the drill string operations are conducted within the wellbore.